1. Field of the Invention
The present invention relates to a free space optical communication link and, in particular, the invention relates to techniques to spread error sources over time intervals and provide redundant channels to reduce the effects of fading.
2. Description Of Related Art
Known optical communication systems rely on optical fibers between transmitter and receiver. However, to establish a system network requires obtaining right of ways and installation of fiber, a time consuming and expensive process.
Free space optical communication systems are fundamentally different than fiber optic systems. Distances are more limited. The media is air and subject to atmospheric disturbances such as fog, rain and resulting fades.
Koh and Davidson (xe2x80x9cInterleaved Concatenated Coding For The Turbulent Atmospheric Direct Detection Optical Communication Channelxe2x80x9d, IEEE Transactions On Communications, Vol. 37, No. 6, June 1989, pages 648-651) discuss how the direct detection atmospheric optical communication channel is characterized by strong fading of the received laser light intensity caused by random variations in the index of refraction encountered by laser light variations as it propagates through the channel.
In addition, the Jet Propulsion Laboratory of the California Institute of Technology published a xe2x80x9cTechnical Support Package on Multiple-Beam Transmission For Optical Communicationxe2x80x9d in November 1998 as NASA Tech Brief, Vol. 22, No. 11 from a JPL New Technology Report NPO-20384. This NASA Tech Brief describes how superposition of mutually incoherent beams would reduce deleterious effects of atmospheric turbulence.
It is an object to the present invention to provide a free space optical communication link that reduces the effects of fading.
This and other objects are achieved in a free space communication system that includes first and second terminals, and the first terminal includes a transmitter to transmit an input signal. The transmitter has plural laser sources, at least one optical delay line and a telescope. Each laser source modulates the input signal onto a wavelength that is distinct from a wavelength of each other laser source. The plural laser sources include a first laser source and at least one other laser source; and the at least one optical delay line is coupled to a respective output of the at least one other laser source. The telescope projects an output of the first laser source and an output of each of the at least one optical delay line toward the second terminal.
In an alternative embodiment, this and other objects are achieved in a free space communication system that includes first and second terminals, the first terminal including a transmitter to transmit an input signal. The transmitter has at least one electrical delay line, plural laser sources and a telescope. The at least one electrical delay line delays the input signal. The plural laser sources include a first laser source and at least one other laser source; the first laser source modulates the input signal onto a first wavelength, and each of the at least one other laser source modulates an output of a corresponding electrical delay line onto a wavelength that is distinct from the first wavelength. The telescope projects an output of each of the plural laser sources toward the second terminal.
The receiver includes diversity reception means to optimally combine the received signals.